The objectives of this study were to test the hypothesis in awake dogs that during control conditions endocardial vessels are maximally dilated and to determine whether variables introduced by general anesthesia and thoracotomy modify distribution of myocardial blood flow or impair capacity for augmentation of flow in response to a coronary vasodilator stimulus. Myocardial blood flow was measured in relatively small, 2 to 3 g, left ventricular epicardial and endocardial samples by using 7 to 10 μm radioisotope labeled microspheres during control conditions and during infusion of adenosine in dosages which produced maximum increases in coronary blood flow. Measurements were made initially in awake resting animals and were repeated after pentobarbital anesthesia, thoracotomy, and pericardiotomy. Blood flow (mean ± SEM) in the epicardium and endocardium, respectively, was 0.75 ± 0.06 and 0.83 ± 0.06 during control conditions and 4.98 ± 0.28 and 4.49 ± 0.27 cm3/min/g during adenosine. These data demonstrate considerable capacity for vasodilation in both myocardial layers and thus refute the hypothesis that endocardial vessels are maximally dilated during control conditions. During control conditions blood flow within epicardial and endocardial layers was essentially homogeneous around the circumference of the left ventricle. In contrast to previous studies in anesthetized animals, however, transmural gradients were present in most regions, i.e., endocardium:epicardium ratio (endo/epi) 1.06 to 1.16. During adenosine, circumferential epicardial flows were homogeneous; however, circumferential endocardial flows were inhomogeneous and increased less than epicardial flows, endo/epi 0.81 to 0.99. Anesthesia, thoracotomy, and pericardiotomy increased epicardial and endocardial flow, mean values 1.08 ± 0.10 and 1.11 ± 0.08 cm3/min/g, respectively. Transmural gradients remained in only papillary muscle regions. Adenosine increased epicardial flow comparably before and after anesthesia. Although adenosine increased endocardial flow 3 to 4 fold after anesthesia, the increase was considerably less than epicardial flow, i.e., endo/epi 0.63 to 0.78.